Yes, it is almost always possible to change your seat after you have completed online check-in, provided the flight is not fully booked. Most major airlines, including United, Delta, and British Airways, allow you to go back into the "Manage Booking" or "Check-In" section of their app or website even after your boarding pass has been issued. From there, you can view the live seat map and move to any available seat. If you find a better spot—such as an exit row that was previously occupied or a seat closer to the front—you can select it and the system will automatically update your digital boarding pass. Some airlines may charge a fee if you move from a standard seat to a "preferred" or extra-legroom seat during this window. If the app doesn't allow the change, you can also ask a gate agent once you arrive at the airport; they often have control over "blocked" seats that were held for operational reasons and can move you right before boarding begins.